1. Technical Field
Embodiments relate to a method for forming a back-side illuminated image sensor. It also aims at a sensor formed according to this method.
2. Discussion of the Related Art
FIG. 1 is a cross-section view schematically and partially showing a back-side illuminated image sensor 1. To form such a sensor, it is started from a semiconductor substrate of standard thickness, for example, a few hundreds of μm, which is thinned from its rear surface to provide a substrate 3 having a thickness ranging from a few micrometers to some ten micrometers. The initial substrate may be a substrate of semiconductor-on-insulator type, a solid silicon substrate possibly coated with an epitaxial layer, or any other type of semiconductor substrate capable of being thinned from its rear surface. In the present example, substrate 3 is of type P.
Before the thinning step, insulating regions 5 forming vertical partitions are formed in the upper portion of the substrate. Regions 5 extend in the substrate, from its front surface and perpendicularly to this surface, down to an intermediary depth. In top view (not shown), regions 5 delimit a plurality of rectangular substrate portions 3a and 3b. Each portion 3a is intended to comprise at least one photodiode and may comprise charge transfer devices (not shown), corresponding to a pixel of the sensor, and each portion 3b is intended to comprise one or several control transistors (not shown). To form insulating regions 5, openings in the form of trenches are etched into the substrate down to the desired depth, and filled with an insulating material such as silicon oxide. After the etching, but before the filling, dopant elements are implanted from the internal walls of the trenches, to create, at the interface between substrate 3 and insulator 5, a thin layer 7 of same conductivity type as the substrate but of higher doping level. Layer 7 especially enables to limit so-called dark currents. Such parasitic currents are due to the spontaneous random generation of electron-hole pairs at the level of certain defects of the crystal structure of the substrate. They are capable of appearing and of being collected by photodiodes even in the absence of any illumination of the sensor. Such currents disturb the sensor operation. In particular, at the interface between substrate 3 and insulator 5, crystal defects created on opening of the trenches are capable of generating dark currents.
The provision of layer 7 enables to both strongly decrease the electron generation rate close to the interface, and increase the probability, when an electron is generated close to the interface, for the latter to be recombined with a hole and thus not to be collected by a photodiode.
After the forming of insulating regions 5, photodiodes and charge transfer devices (not shown) are formed in the upper portion of substrate portions 3a, and control transistors (not shown) are formed inside and on top of substrate portions 3b. The control transistors of a substrate portion 3b may be shared between several neighboring photodiodes, for example between four photodiodes associated with four neighboring substrate portions 3a. 
The front surface of substrate 3 is then coated with a stack 9 of insulating and conductive layers in which the various sensor interconnections are formed. In particular, each substrate portion 3a or 3b is topped with a biasing contacting area 11 formed in stack 9. Each area 11 contacts a P-type region 13 of higher doping level than the substrate, formed in the upper portion of the corresponding substrate portion 3a or 3b. The interconnection tracks and vias, other than those forming areas 11, have not been shown in FIG. 1.
After the forming of stack 9, a holding handle (not shown) is bonded to the upper surface of the sensor, and the substrate is thinned from its rear surface to reach insulating regions 5. As an example, the thickness of substrate 3 remaining after the thinning ranges from 1 to 10 μm. Substrate portions 3a, 3b are then totally insulated from one another by insulating regions 5. In operation, the biasing of substrate portions 3a and 3b to a common reference voltage will be provided via contacting areas 11.
After the thinning, a step of implantation of dopant elements from the rear surface of substrate 3 is provided, to form a layer 15 of same conductivity type as the substrate but of greater doping level. Layer 15 extends from the thinned surface (rear surface) of the semiconductor substrate, across a thickness ranging from 50 to 200 nm. It has the function of limiting dark currents due to the inevitable presence of crystal defects at the level of the rear surface of substrate portions 3a and 3b. Layer 15 is discontinuous, and stops at the level of insulating regions 5.
After the forming of layer 15, a laser surface anneal of the rear surface of the substrate is provided to stabilize this surface. A thin insulating layer 17, for example, a silicon oxide layer with a thickness of a few nanometers, is then formed on the rear surface of substrate 3. Layer 17 is itself coated with an antireflection layer 19, for example formed of a stack of several transparent dielectric layers of different indexes. Antireflection layer 19 is topped with juxtaposed color filtering elements, altogether forming a layer 21. In the shown example, a first substrate portion 3a containing a first photodiode is topped with a green filtering element (G) and a second substrate portion 3a containing a second photodiode is topped with a blue filtering element (B). Microlenses 23 are formed on top of filtering layer 21, in front of substrate portions 3a. 
A first disadvantage of this type of sensor is the need to provide an implantation of dopant elements from the rear surface of the substrate after the thinning step, to form layer 15. At this stage of the manufacturing, the front surface of the sensor is already coated with a stack of insulating and metallic layers. There thus is a risk of contamination of the implantation equipment by the interconnection metals arranged on the front surface (for example, copper). In practice, this forces to use implantation equipment specifically dedicated to the forming of layer 15, separate from the equipment already provided to perform implantations from the front surface of the substrate.
Another disadvantage of this type of sensor is that the thickness of layer 15 is relatively large and poorly controlled. This adversely affects the sensor sensitivity, especially for wavelengths for which the photons are absorbed by very small silicon thicknesses (blue or ultraviolet). If layer 15 is too thick, photons may be absorbed in this layer. Now, this layer is precisely provided to limit the generation of electrons (to decrease dark currents). This results in a decrease in the sensor sensitivity for such wavelengths.
Another disadvantage of such a sensor is the significant surface area taken up by biasing contacting areas 11 and by the corresponding silicon regions 13. Such areas are necessary to provide, in operation, the biasing of substrate portions 3a and 3b to a common reference voltage, but their presence increases the total silicon surface area necessary to form the sensor. Further, the provision of areas 11 and of regions 13 in substrate portions 3a containing photodiodes tends to increase dark currents in the sensor.